In geothermal wells, water wells, or some oil and has wells, the final drilled interval where production occurs is completed by hanging a liner such as a slotted or perforated casing string or a manufactured well screen from the last cemented casing string above. This last liner in those wells is not cemented in place. Instead, the open annulus between the last liner and the open wellbore is left open or sometimes is packed with gravel. This type of well completion technique stabilizes the production interval of the formation by leaving the maximum area open to the wellbore and reducing pressure drop as fluids enter the wellbore. Resultantly, the flow rate of fluids is increased to the well and the recovery of fluids during production is improved.
While this type of well completion technique reduces the pressure drop from flow into the well and improves production, it makes difficult and sometimes impossible some types of work on the well that are to be performed after the completion. For example, intervals behind the slotted or perforated liner or well screen are difficult to isolate for sealing a desired zone or stimulating zones deeper in the well. In an oil and gas well, a zone that has been produced may start to produce an increased flow of water or fluid injected to enhance oil or gas recovery such as steam, CO2, water or other fluid. The water or other fund may breakthrough in a particular zone. In geothermal wells, shallow zones that are productive may have cooler temperature fluids than expected, or cool injected water may enter the wellbore in the open interval.
Generally, wells that require stimulation may have a cemented casing at a shallower zone than needed to stimulate zones behind the slotted or perforated liner or well screen. This may prevent the build-up of pressures required to stimulate deeper zones because fracturing will occur in the shallow zones. Therefore, the maximum hydraulic pressure that can be applied in the stimulation treatment is limited to the fracture breakdown pressure at the depth of the last casing shoe. The limited hydraulic pressure hampers or disables stimulation of formation deeper in the open hole interval of the well. The potential for fluid production improvement, thus the economic value of the asset is compromised.
Sometimes, a packer is set in the slotted or perforated liner or a well screen, and cement is pumped into the liner above the packer. However, cement is denser than water, therefore cement flows down the annulus between the slotted liner and the wellbore, and enters permeable zones deeper in the well. The intrusion of cement into permeable zones needs to be avoided because this impairs production from these zones.